linux-kernel - Re: [PATCH v2] sched/fair: Fix unfairness caused by stalled tg_load_avg

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAKfTPtA_czLC=EEeBvvKQzGv58v6NNk=z2qikjVNj6XrxyDG1Q@mail.gmail.com>
Date: Tue, 5 Aug 2025 18:10:21 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: xupengbo <xupengbo@...o.com>
Cc: ziqianlu@...edance.com, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Aaron Lu <aaron.lu@...el.com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org, 
	cgroups@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Fix unfairness caused by stalled
 tg_load_avg_contrib when the last task migrates out.

On Tue, 5 Aug 2025 at 16:42, xupengbo <xupengbo@...o.com> wrote:
>
> When a task is migrated out, there is a probability that the tg->load_avg
> value will become abnormal. The reason is as follows.
>
> 1. Due to the 1ms update period limitation in update_tg_load_avg(), there
> is a possibility that the reduced load_avg is not updated to tg->load_avg
> when a task migrates out.
> 2. Even though __update_blocked_fair() traverses the leaf_cfs_rq_list and
> calls update_tg_load_avg() for cfs_rqs that are not fully decayed, the key
> function cfs_rq_is_decayed() does not check whether
> cfs->tg_load_avg_contrib is null. Consequently, in some cases,
> __update_blocked_fair() removes cfs_rqs whose avg.load_avg has not been
> updated to tg->load_avg.
>
> I added a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
> which blocks the case (2.) mentioned above. I follow the condition in
> update_tg_load_avg() instead of directly checking if
> cfs_rq->tg_load_avg_contrib is null. I think it's necessary to keep the
> condition consistent in both places, otherwise unexpected problems may
> occur.
>
> Thanks for your comments,
> Xu Pengbo
>
> Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
> Signed-off-by: xupengbo <xupengbo@...o.com>
> ---
> Changes:
> v1 -> v2:
> - Another option to fix the bug. Check cfs_rq->tg_load_avg_contrib in
> cfs_rq_is_decayed() to avoid early removal from the leaf_cfs_rq_list.
> - Link to v1 : https://lore.kernel.org/cgroups/20250804130326.57523-1-xupengbo@oppo.com/T/#u
>
>  kernel/sched/fair.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..a35083a2d006 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4062,6 +4062,11 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>         if (child_cfs_rq_on_list(cfs_rq))
>                 return false;
>
> +       long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> +
> +       if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64)

I don't understand why you use the above condition instead of if
(!cfs_rq->tg_load_avg_contrib). Can you elaborate ?

strictly speaking we want to keep the cfs_rq in the list if
(cfs_rq->tg_load_avg_contrib != cfs_rq->avg.load_avg) and
cfs_rq->avg.load_avg == 0 when we test this condition


> +               return false;
> +
>         return true;
>  }
>
> --
> 2.43.0
>